A common class of device for detecting electrical faults in a remote system involves sensing the current flowing to the remote system and the current flowing back from the remote system to see if they are the same. In certain types of such arrangements, the over-all circuit is what is commonly designated as a series loop, in that current is supplied from a central station to a remote measuring apparatus, whence it flows back to the central station, with any difference between the outgoing and returning currents representing a fault, such as a flow to ground of current within the remote system or in the leads extending to or from it.
One form of apparatus intended to effect such fault detection is described in U.S. Pat. No. 4,371,832 of Gerald L. Wilson, issued Feb. 1, 1983. In that apparatus the current flowing out to a remote monitoring device is passed through a coil on a toroidal magnetic core and the return current is passed through another coil on the same core, but in the opposite relative direction with respect to the core. If there is no fault present the two currents will be equal, and will produce no net magnetization of the core; however, should there be a fault, these two currents will differ and will produce an imbalance resulting in a net magnetization of the core in one direction or the other, depending on which current is the larger; typically, a ground fault causes the return current to be smaller than the outgoing current.
The U.S. Pat. No. 4,371,832 cited above is concerned with apparatus for detecting such imbalances when they occur. To accomplish this detection, an excitation coil is provided on the same core as that on which the outgoing current coil and the incoming current coil are located, to which excitation coil an excitation voltage of alternating polarity is applied; the applied voltage is caused to reverse at the times when the current through the excitation coil causes the core to reach a magnetic saturation level in one direction or the other. The frequency or duty cycle of the excitation voltage is then measured, and utilized as an indication of current balance or imbalance, and hence of the absence or presence of a fault.
While useful for certain purposes, the system of the U.S. Pat. No. 4,371,832 patent does suffer from the drawback that the system output is in the form of a pulse train of varying frequency, complicating the subsequent digital circuitry which interprets the output of the system.
It is desirable to provide a fault detector system in which the input excitation voltage and the output pulse voltages are all of the same common frequency, simplifying the subsequent digital circuitry which interprets the output of the system.